The invention relates to a stacker for stacking products that are die-cut from a continuous web by a die cutter into vertical stacks. The stacker is of the kind that comprises at least one stacking chute having an upper opening and a number of mainly vertically arranged walls, at least one overlying belt conveyor having a conveyor belt designed with a number of suction holes distributed along the belt and having a lower run with a direction of conveyance running from the die cutter to the stacking chute, and a suction box located above the lower run and connected to a vacuum source communicating with the suction holes of the lower run via the suction box in an area around the die cutter but not in an area around the stacking chute.
Such products can e.g. be labels which are affixed in a labelling plant with magazine. During operation, the magazine is refilled with labels supplied by the labels manufacturers in handy heights.
In some cases, the labels are die-cut from a plurality of sheets having the same height as the desired stack. For this purpose a tubular die cutter is employed. However, this method of die-cutting has the effect of the labels in the same stack not being completely uniform and the die cutter furthermore leaves the labels with a rather irregular edge.
Instead, rotary die cutters have been widely used for die-cutting labels. In a rotary die cutter, a web of labels are continuously conveyed between two counter-rotating rollers. One of the rollers is a plain impression roller and the other is a die-cutting roller having cutting edges for die-cutting the labels.
By means of this method, very high production rates are obtained, and the die-cut labels are furthermore uniform and have a regular and sharp edge. In many cases, the die-cutting roller is designed with a cutting edge for being able to parallel die-cut several labels at the same time.
The web of labels is often made of a dielectric material which is easily charged with static electricity during conveying. The same is the case for the die-cut labels which are normally conveyed on to a stacking station by means of one or several conveyor belts during which the labels are sliding on the belt and/or each other.
The statically charged labels are inclined to stick to e.g., the rolls or frame of the rotary die cutter, and they repel or attract each other in such a way that they are difficult or often impossible to place in an orderly and accurate stack.
Therefore, methods have been developed for discharging the labels by means of e.g., ionized air, brushes or carbon bars. However, these methods are difficult to work with and furthermore require that the labels are conveyed at a relatively low rate, the result of which is that the capacity of the rotary die cutter is far from being utilized to a satisfactory extent.
Thus, there is a need for improvements in such devices.
With a view towards solving the problems mentioned above, the present invention now provides a stacker which has a simple structure and which is able to stack products successively die-cut from a continuous web at higher rates than hitherto known.
The stacker generally comprises at least one stacking chute having an upper opening and at least front and back vertically arranged walls, at least one overlying belt conveyor that includes a conveyor belt having a number of suction holes distributed along the belt, and a lower run with a direction of conveyance running from the die cutter to the stacking chute, with the front wall of the stacking chute being arranged as the first wall to be encountered by the lower run of the belt in the direction of conveyance, and a suction box located above the lower run and connected to a vacuum source communicating with the suction holes of the lower run of the belt via the suction box in an area around the die cutter but not in an area around the stacking chute.
In one embodiment of the invention, novel and unique features are achieved by the addition of a channel for receiving all or a lower part of the lower run of the conveyor belt. This channel is provided in at least a front wall of the walls of the stacking chute. A product, such as a label or other sheet material, which is conveyed through the front channel is thereby bent in such a way that its diameter is reduced whereby the product is allowed to fall into the stacking chute without hindrance. When the front channel continues into a corresponding channel in the back wall, the latter wall will function as stop to effectively position the labels in the stacking chute.
The products are safely caught in the stacking chute when the walls have a top face inclining downwards from the back to the front wall and continuing into an inclined inlet face extending down under the lower run of the conveyor belt. Thus, the stacking chute walls provide a top opening for the chute that inclines downwardly from the back wall to the front wall, with the top opening continuing into an inclined inlet face or shelf that extends beneath the lower run of the conveyor belt. During the fall down through the stacking chute, the products will meet flow resistance from the air under the products. To reduce this resistance as much as possible, apertures can be provided in the walls of the stacking chute, through which air can exit.
During conveyance from the die cutter to the stacking chute, the products are hanging on the underside of the lower run of the belt conveyor without thereby contacting each other or slide along the conveyor belt. In this way the possibility of the products unintentionally being charged with static electricity is eliminated. The products can therefore be stacked at high rates in orderly and accurate stacks in the stacking chute.
Conventionally, the products made of a dielectric material which discharges the static electricity with which they are charged during conveyance in the plant. However, this process reduces the rate by which it is possible to stack the labels.
According to another feature of the invention, the invention the products can be charged electrostatically prior to and/or during stacking in such a way that the opposite sides of each product will obtain different polarities, and that the sides having the same polarity are facing in the same direction on all products.
The static electricity is now utilized for increasing the stacking rate by positively getting the products to attract each other during stacking. Another advantage is that the complete stack is bound closely and firmly by the electrostatic forces acting between the stacked products.
To avoid that the products are charged with static electricity owing to the fact that they rub against each other during conveyance from the die cutter to the stacking chute, the rate of advance of the conveyor belt can be higher than the production rate of the die cutter whereas the spacing between two adjacent suction holes or set of suction holes in the conveyor belt can be greater than the extent of the products in the direction of conveyance of the belt conveyor.
When a suction shoe is designed around each of the suction holes of the conveyor belt and is extending out from the rest of the belt, the products are kept at a distance from this belt and are thereby ensured against unintentional charging with static electricity due to contact with the belt. In some cases, it is sufficient with one suction hole for each product. In other cases, it would be an advantage to have two or more suction holes for each product. Thereby the products are better and more safely controlled during conveyance.
Preferably, the conveyor belt conveys the products at a higher rate than the production rate of the die cutter. Thereby, the products are kept apart from each other during conveyance. However, it will be an advantage if the products are kept at the same mutual spacing even though the production rate of the die cutter is increased or reduced. For this purpose, the stacker can have a servo-control for controlling the rate of the belt conveyor at a fixed relation in dependence on the production rate of the die cutter.
The invention also relates to a method for stacking labels which comprises bending the labels before they enter the stacking chute to assist in stacking the labels in the chute. The channel conveniently bends the labels as the lower part of the conveyor moves them past the front wall of the stacking chute.